Various types of clips and clamps have been employed in the past to releasably attach the leading edge of a flexible sheet or web to a rotary roll or drum so that the sheet or web is wrapped about the roll when the roll is rotated. Such mechanisms have been incorporated into the rolls of duplicating machines, printers and diverse winding apparatus. For example, U.S. Pat. No. 4,388,628 discloses a thermal transfer recorder for printing on a recording sheet. The leading edge of the recording sheet is attached to the roll, apparently manually, by means of a clip and the roll is rotated to wrap the sheet about the roll. During successive rotation cycles of the roll, subtractive color inks are applied to the sheet in register to form color printing thereon. Following completion of the printing operation, stripper fingers strip the printed sheet from the roll. However, no mechanism is disclosed there for opening and closing the clip at the beginning and end of each printing operation.
U.S. Pat. No. 4,346,856 discloses a device for rewinding material delivered in sheets in a heliographic machine or the like. That device includes a hollow rotary casing provided with a longitudinal slit. The casing is rotatively mounted on a driven shaft provided with radially-extending tabs along its length. The casing remains stationary until the leading edge of the sheet of material to be wound is inserted into the casing slot. As soon as the insertion takes place, the tabs on the internal driven shaft press that edge against a slotted plate on the inside wall of the casing, thereby clamping that edge to the casing while at the same time pulling around the casing so that the sheet is wound about the casing. After the driven shaft is stopped, the casing is temporarily removed from the shaft and the roll slid endwise from the casing. Thus, this type of device clamps the sheet only when there is relative rotation between the internal shaft and the casing. It is basically an inertial device whose clamping action bears no particular relationship to the angular position of the casing.
U.S. Pat. No. 1,870,243 shows a rotary winding block for winding strips of sheet material. This block also relies on relative motion between the block and an internal shaft to clamp the leading edge of the sheet to the block. Therefore, its clamping action is not dependent upon the angular position of the block.
Finally, U.S. Pat. No. 4,199,116 discloses a coiler or winder incorporating a roll around which strips of sheet material are wound. The roll includes an internal gripper bar extending the length of the roll which is accessible through a lengthwise slot in the roll wall. When the leading edge of the sheet material is inserted into the slot, it is clamped in place by the operator turning the gripping bar manually by means of a tab on the end of the bar. When the roll is rotated, the sheet material is coiled about the roll. When coiling is completed, a trip member positioned adjacent the roll is actuated so that, when the roll revolves to a selected angular position, the tab is engaged by the trip member, thereby rotating the gripper bar to its release position so that the coil of sheet material can be slid axially from the roll. Thus, in this clamping mechanism, no provision is made for automatically clamping the leading edge of the sheet material to the roll. Also, the roll only rotates in a sheet-material winding direction and the sheet material is released or unclamped when the roll is rotating in that same direction. Another clamping mechanism of this general type is disclosed in the old U.S. Pat. No. 297,739.
Thus, the prior devices for clamping or clipping sheet material to rotatable rolls and drums are not satisfactory for use in present-day color printers in order to automatically receive successive sheets of paper from a feed path and to clamp the leading edge of each sheet to a drum while the drum makes a plurality of revolutions as the sheet is being printed on and then to automatically release and eject the printed sheet to a discharge path prior to receiving the next sheet to be printed on. Those prior clamping mechanisms which do provide some automatic functions are complex devices requiring various gears and levers inside the body of the winding roll. This makes them relatively expensive to make and maintain. Also, some of the above-described prior clamping devices used for coiling metal strips clamp an excessive amount of the sheet material and sometimes distort that material. Accordingly, if they were incorporated into the drums of printing apparatus, they would disfigure the printed sheets.